System and method for processing a periodic or cyclostationary signal

ABSTRACT

In one representative embodiment, multiple ensembles of samples of a periodic or cylcostationary signal are processed in a time aligned manner. The sampling rate of the processing system is adjusted so that an integer number of sampling intervals equals the period of the signal. A cyclic counter is programmed to reset according to the integer number. Also, the cyclic counter may be initialized according to an external trigger. During operation, the cyclic counter is incremented when each sample is received. Continuous operation of the cyclic counter with the capturing of samples enables precise time alignment between ensembles of samples. Specifically, the beginning of a discrete ensemble is identified by a reset of the cyclic counter. Because each ensemble is time aligned, further processing (e.g., coherent averaging) may occur without post-processing to time-shift each sample to achieve the time alignment.

TECHNICAL FIELD

The present invention is generally related to analysis or similarprocessing of period or cyclostationary signals.

BACKGROUND

Instruments such as oscilloscopes, spectrum analyzers, modulationanalyzers, and vector signal analyzers, normally implement a triggermechanism to align the beginning of a viewing and analysis interval witha particular instant in time. This desired trigger time may beidentified by a separate “trigger input” signal. It may also be based onan attribute of the input signal such as the crossing of a particularvoltage or power level. Virtually all of these instruments enable theselection of the type or source of the trigger signal and the relatedsignal attributes such as threshold and polarity.

The precision of the trigger timing is complicated by the fact that mostinstruments convert the input signal to digital form by sampling theinput signal at uniform time intervals. If the desired trigger time doesnot precisely align with one of these samples, there is an error in thetrigger timing. Unless additional measures are taken, the inherenttrigger uncertainty is one period of the instrument's sample clock. Insome cases, this amount of uncertainty can be tolerated. However, thereare applications that require greater precision.

For example, analysis of a periodic signal or a cyclostationary signal(a signal with periodic statistics) typically involves displaying orprocessing ensembles of many signal periods according to accurate timealignment. An ensemble, as used herein, refers to samples of a singleperiod of a signal. An example of analysis of multiple ensembles ofsignal data is coherent averaging of several periods of a signal toreduce measurement noise while retaining the underlying signal. Thetrigger precision requirements for periodic or cyclostationaryprocessing often exceed the inherent limit imposed by an instrument'ssample rate.

Some instruments provide a trigger interpolator which measures the timeinterval between an externally supplied trigger and the next availablesignal sample. The time offset is then used to post-process the captureddata to effectively shift the time alignment to correspond to thedesired trigger time. This technique has been observed to improve theeffective trigger accuracy by a factor of 10 to 100. However, thistechnique involves added complexity for the trigger interpolationfunctionality. Additionally, the post-processing time can besignificant. Furthermore, a degree of inaccuracy remains in the timealignment.

Techniques have been developed to extract high resolution trigger timingby post-processing the data even when an external signal is notsupplied. Because the value of the offset between the sampling timingand the trigger timing is determined from the signal itself, thesetechniques can only be applied in a limited number of special cases.After the offset is determined, the timing offset is compensated usingtime consuming post-processing techniques. However, the extraction ofthe trigger offset can artificially skew the timing and obscure jittereffects that are present in the signal. Also, in low signal-to-noiseconditions, this technique cannot be applied.

SUMMARY

Representative embodiments are directed to systems and methods forfacilitating analysis of a signal. In some representative embodiments,the period of the signal is determined. The period may be defined by theinterval of time between a repetition of respective phases of adeterministic signal. Alternatively, the period may be defined by theinterval of time between a repetition of signal statistics forcyclostationary signals.

The sampling rate of the instrument used to capture signal samples isthen adjusted so that an integer number of samples matches thedetermined period. The sampling rate may be adjusted using a number ofmechanisms. For example, the digital clock signal that is used tocontrol an analog-to-digital (A/D) converter may be varied using aprogrammable frequency synthesizer. Alternatively, the A/D converter maybe operated at its optimal rate and the samples from the A/D convertermay be processed by an arbitrary rate digital resampler.

A cyclic counter with a programmable modulus is configured to resetaccording to the integer number of samples that matches the determinedperiod. Specifically, the cyclic counter returns to zero after theinteger number of samples have been obtained. The A/D conversion and theoperation of the counter mechanism continues even when data samples ofthe signal are not being captured for signal analysis.

An initial trigger event may be used to identify a particular digitalsample as the starting point for an ensemble of measurements. The cycliccounter is initialized to zero at this point. During further operations,samples are captured between respective resets of the cyclic counter.Because of the configuration of the cyclic counter, the captured sampleswithin each ensemble are precisely time aligned. Further processing maythen occur in an efficient manner without requiring post-processing timealignment. For example, coherent averaging may occur by only addingrespective ensemble data samples and appropriate scaling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a system that captures multiple ensembles of data in atime aligned manner according to one representative embodiment.

FIG. 2 depicts another system that captures multiple ensembles of datain a time aligned manner according to one representative embodiment.

FIG. 3 depicts a flowchart for capturing multiple ensembles of data in atime aligned manner according to one representative embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, FIG. 1 depicts system 100 that capturesmultiple ensembles of data in a time aligned manner according to onerepresentative embodiment. As shown in system 100, signal 110 isreceived by analog-to-digital (A/D) converter 101. The sampling intervalof A/D converter 101 is defined by a timing signal provided byprogrammable frequency synthesizer 102. Also, the timing signalgenerated by programmable frequency synthesizer 102 is provided tocyclic counter 103 to increment its counter value. Cyclic counter 103possesses a programmable modulus (the value at which cyclic counter 103resets to zero). Cyclic counter 103 may be implemented using any numberof digital logic designs.

Controller 109 coordinates the operations of the various components ofsystem 100. Controller 109 may be implemented using a processor andsuitable executable instructions. Alternatively, controller 109 may beimplemented using an application specific integrated circuit if desired.When samples of signal 110 are to be captured, the period of signal 110is identified to controller 109 by analysis or using a priori knowledge.Controller 109 sets the frequency of program frequency synthesizer 102such that A/D converter 101 samples an integer number of samples foreach period of signal 110. Also, controller 109 sets the programmablemodulus of cyclic counter 103 to cause a reset after the integer numberof samples are captured.

For example, suppose system 100 possesses a nominal sampling rate of 10MHz. Also, assume that signal 110 is a cyclostationary signal with aperiod of 1/60 second. This would result in approximately 166666.66samples per period. Accordingly, a small adjustment can be made to thesample rate to achieve exactly 1666667 samples per second. Controller109 determines the appropriate frequency adjustment and provides theadjustment to programmable frequency synthesizer 102. Also, controller109 calculates the integer number of samples obtained during the periodof signal 110 and loads the number into cyclic counter 103 to set themodulus. Cyclic counter 103 updates its counter in response to thetiming signal received from programmable frequency synthesizer 102 andcyclically resets to zero after every 1666667 samples.

Initialization functionality 105 may be used to identify a particularsample as the sample to begin an ensemble. Initialization functionality105 may be implemented using a typical trigger mechanism. Additionallyor alternatively, initialization functionality 105 may receive userinput and, in response thereto, advance or delay the trigger point(i.e., the sample identifying the beginning of an ensemble). The usercan then “rotate” through the periodic signal samples until the desiredalignment is obtained.

During operation, signal 110 is sampled by A/D converter 101. Cycliccounter 103 counts the number of samples, because it is coupled toprogrammable frequency synthesizer 102. When cyclic counter 103 resetsits counter, it generates a trigger message for communication forcapture circuitry 104. Capture circuitry 104 beings reading out adiscrete ensemble of samples from A/D converter 101. The ensemble ofdata samples can then be subjected to further processing. For example,the ensemble of data samples can be provided to coherent averagingcircuitry 106. Because the sample of the various ensembles are preciselytemporally aligned, coherent averaging may occur in an efficient mannerwithout requiring individual post-processing of each sample.

The coherently averaged samples can then be stored in memory 107 forlater retrieval, used to display signal characteristics on display 108,and/or the like. Additionally, if a temporary cessation in samplecapture occurs to perform other operations, cyclic counter 103 continuesits operations to maintain synchronization to signal 110. Accordingly,when sample capturing resumes, the subsequent ensembles will be properlytime aligned with prior ensembles.

FIG. 2 depicts system 200 that captures multiple ensembles of data in atime aligned manner according to another representative embodiment.System 200 operates largely in the same manner as system 100. However,in lieu of using programmable frequency synthesizer 102 to control thesampling interval of A/D converter 101, A/D converter 101 is operatedusing a single sampling interval. By doing so, A/D converter 101 may beoperated at its optimal rate. In this case, the number of capturedsamples for a period of signal 110 are caused to equal the desiredinteger number by employing digital resampler 201. Digital resampler 201enables a programmable reduction in the sampling rate of an input datastream. Controller 109 sets the resampling rate to achieve an integernumber of samples per signal period. Also, in system 200, cyclic counter103 is coupled to digital resampler 201 to enable the reset of cycliccounter 103 after each integer number of resampled samples are providedto capture circuitry 104.

System 200 may be appropriate for processing signals that possessrelatively short signal periods. Specifically, short periods may causethe variation in the sample interval to cause operation of A/D converter101 outside of its optimal region. Accordingly, the performance ofdigital resampling maintains the performance of the analog-to-digitalconversion while obtaining the desired number of samples for timealignment purposes. Additionally, some digital resamplers enable thesample times to be increased or decreased by a fraction of theprogrammed sample interval. In conjunction with the cyclic counteradjustments, representative embodiments may use this feature of digitalresamplers to obtain any desired degree of resolution in the timealignment of the cyclic counter initiated trigger relative to the inputsignal.

FIG. 3 depicts a flowchart for capturing multiple ensembles of data in atime aligned manner according to one representative embodiment. In step301, the period of the signal is identified. The period may define aninterval between a repetition of a phase of said signal. Alternatively,the period may define an interval between a repetition of signalstochastic properties. In step 302, the sample rate of the processingsystem is adjusted so that an integer number of sample intervals equalsthe period of the signal. The adjustment may be performed using aprogrammable frequency synthesizer to adjust a clock signal provided toan A/D converter. Alternatively, the adjustment may be performed using aprogrammable rate digital resampler. The timing of said sample rate mayalso be adjusted by a fraction of the period of the sample interval toachieve a desired level of resolution.

In step 303, a cyclic counter is configured to reset according to theinteger number. Also, the cyclic counter may be initialized using aconventional trigger mechanism. Alternatively, a reset point of saidcyclic counter may be manually or otherwise adjusted to delay or advancesaid trigger signal relative to the periodic signal. Each reset of thecyclic counter may be used to generate a trigger signal to initiate thecapture of a respective ensemble. In step 304, ensembles of data samplesof the signal are captured. Specifically, each ensemble is defined bysuccessive resets of said cyclic counter. In step 305, additionalprocessing of ensembles may occur such as coherent averaging.

By adjusting the sampling interval and continuously operating the cycliccounter to count captured samples, ensembles of data samples aremaintained in a time aligned manner according to representativeembodiments. Because post-processing time shifting is not required,hardware averaging or other suitable processing may be employed toprocess the ensembles in an efficient manner. Additionally, theprecision of the alignment accuracy achieved by representativeembodiments is only limited by the jitter limitations of the ADCsampling, which is typically in the range to 10⁻¹² seconds. Accordingly,some representative embodiments may enable alignment accuracy to bemaintained for hours or days without requiring another traditionaltrigger to correct the alignment.

1. A method for facilitating analysis of a signal, comprising:identifying a period of said signal; adjusting a sample rate so that aninteger number of sample intervals equals said period of said signal;configuring a cyclic counter to reset according to said integer number;and capturing ensembles of data samples of said signal that arerespectively defined by successive resets of said cyclic counter.
 2. Themethod of claim 1 further comprising: coherently averaging capturedensembles without post-processing to time shift each sample of saidcaptured ensembles.
 3. The method of claim 1 wherein said adjusting isperformed using a programmable frequency synthesizer.
 4. The method ofclaim 1 wherein said adjusting is performed using a programmable ratedigital resampler.
 5. The method of claim 1 further comprising:generating a trigger signal for said capturing in response to a reset ofsaid cyclic counter.
 6. The method of claim 5 further comprising:adjusting a reset point of said cyclic counter to delay or advance saidtrigger signal relative to said signal.
 7. The method of claim 1 furthercomprising: initializing said cyclic counter using a trigger mechanism.8. The method of claim 1 further comprising: adjusting timing of saidsample rate by a fraction of a period of said sample interval.
 9. Themethod of claim 1 wherein said period of said signal defines an intervalbetween a repetition of a phase of said signal.
 10. The method of claim1 wherein said period of said signal defines an interval between arepetition of signal stochastic properties.
 11. A system for processinga signal that possesses a period, comprising: an analog-to-digital (A/D)converter for sampling said signal; sample capture circuitry forobtaining samples of said signal at a sample rate such that an integernumber of sample intervals equals said period; cyclic counter logic thatcounts each captured sample and comprising a programmable modulus set tosaid integer number; and wherein said sample capture circuitry isoperable to output time aligned ensembles of captured samples that aredefined by respective resets of said cyclic counter logic.
 12. Thesystem of claim 11 further comprising: a programmable frequencysynthesizer for varying a sampling rate of said A/D converter.
 13. Thesystem of claim 11 further comprising: digital resampling circuitrydisposed between said A/D converter and said sample capture circuitryfor programmably varying a rate that samples are provided to said samplecapture circuitry.
 14. The system of claim 13 wherein said digitalresampling circuitry adjusts timing of communication of samples betweensaid A/D converter by a fraction of a programmably variable rate of saiddigital resampling circuitry.
 15. The system of claim 11 furthercomprising: a trigger mechanism configured to initialize said cycliccounter logic.
 16. The system of claim 11 wherein said period defines arepetition between a phase of said signal.
 17. The system of claim 11wherein said period defines a repetition between signal stochasticproperties.
 18. A system for processing a signal that possesses aperiod, comprising: means for providing samples of said signal at asample rate, wherein an integer number of sample intervals defined bysaid sample rate equals said period; means for cyclically countingsamples provided by said means for providing such that said means forcyclically resets after said integer number; and means for outputtingtime aligned ensembles of samples defined by respective resets of saidmeans for cyclically counting.
 19. The system of claim 18 wherein saidmeans for providing samples comprises: a programmable rate digitalresampler.
 20. The system of claim 18 wherein said means for providingsamples comprises: a digital to analog converter driven by a clockdefined by a programmable frequency synthesizer.